1. Field of Invention
This invention relates to semiconductor light emitting devices such as light emitting diodes and, in particular, to growth substrates on which such light emitting devices may be grown.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
Since native III-nitride substrates are generally expensive and not widely available, III-nitride devices are often grown on sapphire or SiC substrates. Such non-III-nitride substrates are less than optimal for several reasons.
First, sapphire and SiC have different lattice constants than the III-nitride layers grown on them, causing strain and crystal defects in the III-nitride device layers, which can cause poor performance and reliability problems.
Second, in some devices it is desirable to remove the growth substrate, for example to improve the optical properties of the device or to gain electrical access to semiconductor layers grown on the growth substrate. In the case of a sapphire substrate, the growth substrate is often removed by laser dissociation of the III-nitride material, typically GaN, at the interface between the sapphire and the semiconductor layers. Laser dissociation generates shocks waves in the semiconductor layers which can damage the semiconductor or contact layers, potentially degrading the performance of the device. Other substrates may be removed by other techniques such as etching.